how do I make an object bend or break if hit with sufficient force?
Unfortunately, dynamic fracturing/destruction is not truly possible yet. Though there are some interesting experiments:


Blender Demolition - Proxy Mesh Utilization (Demo 6)
Real-time Shattering Demolition physics in the BGE
BA thread with a patch developed by the author of the above videos


However, you can fake fracturing with the Cell Fracture addon (which is bundled with blender, you can enable it in CtrlAltU> Preferences > Addons > Cell Fracture) and rigid-body contraints:


It's not necessary, but you can get finer control over how an object is fractured by using the Grease Pencil:


Fracture the object into separate objects by pressing Cell Fracture in 3D view > Tool Shelf:


Make them rigid-body objects and constraint them using another addon called Bullet Constraints Tools:


Use the Calculate Mass tool to calculate and set the mass of all selected objects based on volume. I used the Glass (Broken) preset for the shards and the Bronze preset for the monkey:


Manually animate the render visibility of the original and fractured objects in the Outliner (camera icon), so that the original is used until it is fractured by an impact, or use post processing. (see related post)




You can simulate bending with soft-bodies, however tearing is not possible. (though again there are some tests)